High magnetic field study of RuSr₂GdCu₂O₈

.The ruthenate-cuprate RuSr2GdCu2O8 (Ru1212) is a high-Tc superconductor whose crystal structure can be derived from the very well-known structure of YBa2Cu3O7-δ (YBCO) with the substitution Y → Gd, BaO layer → SrO layer, CuO chains → RuO2 planes. Apart from superconductivity at Tc ≈ 50 K, Ru1212 shows also magnetic ordering of the Ru moments at TRu ≈ 130 K and the Gd moments at TGd ≈ 2.5 K. We present magnetization measurements of Ru1212 in pulsed magnetic fields with B ≤ 50 T and investigate the possibility of mixed valency of the Ru ions [60 % Ru5+ (S =3/2) and 40 % Ru4+ (S = 1)]. Within this model, the magnetic ordering of the Ru moments could be realized as the result of a double-exchange interaction between Ru5+ and Ru4+ ions. Superconductivity occurs in the CuO2 layers, but transport of charge carriers, not in a superconducting state, also takes place in the RuO2 planes. Different Ru5+/ Ru4+ ratios can explain many of the contradicting results on Ru1212.
e-mail:T.Papageorgiou@fz-rossendorf.de